1. Field of the Invention
The present invention relates to a negative electrode made of carbon material used for a storage battery including a positive electrode capable of being reversibly charged and discharged and a nonaqueous electrolyte containing lithium salt. More particularly, the present invention relates to a negative electrode for a storage battery having high reliability without the formation of a dendrite. The present invention further relates to a process for producing such a negative electrode.
2. Description of the Related Art
A storage battery with a nonaqueous electrolyte using lithium (Li) as a negative electrode has a high electromotive force. The energy density thereof is, therefore, higher than that of a conventional nickel-cadmium battery or a lead battery.
However, in such a storage battery using metallic Li as a negative electrode, when the battery is charged, the Li metal is deposited on the surface of the negative electrode, forming a dendrite. The dendrite formation often causes a short-circuit within the battery.
In order to prevent the deposition of Li metal at charging, there has been studied using an alloy of Li and aluminum (Al) or Li and lead (Pb), or an intercalation compound of inorganic carbon (C) such as graphite and Li as a negative electrode active material.
As for using the alloy as the negative electrode, since Li can be stored in the alloy at charging, there is no dendrite formation. Thus, a battery with high reliability can be obtained. However, the discharge potential of the negative electrode made of the alloy is higher than that of the electrode made of Li metal by approximately 0.5 V. This means that when the alloy is used as the negative electrode, the discharge voltage of the resultant battery decreases by approximately 0.5 V, and thus the energy density of the battery lowers, compared with when unalloyed Li metal is used.
As for using an intercalation compound of inorganic carbon such as graphite and Li as the negative electrode active material, Li can be stored between layers of carbon at charging. Thus, there is no dendrite formation. Further, the increment of the discharge potential of such a negative electrode from that of the electrode made of Li metal is only approximately 0.1 V. Accordingly, the decrease of the discharge voltage of the resultant battery can be reduced.
The intercalation compound of carbon material such as graphite and Li is represented by C.sub.6 Li, which means that stoichiometrically Li equivalent to one-sixth of the carbon can be stored in the compound at charging. The theoretical capacitance at this time is 372 Ah/kg. However, when the intercalation compound is actually applied to the battery, the capacitance at charging and discharging is only approximately 230 Ah/kg.